A leading cause of lower back pain arises from lumbar intervertebral disc pathology, including rupture or degeneration of the disc. Radicular pain in the lower extremities may be caused by the compression of spinal nerve roots by a bulging disc. Additionally, lower back pain may be caused by collapse of the disc and the dysarthrosis of an unstable or degenerative vertebral facet joint. One proposed method of managing these problems is to remove the problematic disc and replace it with a porous device that restores disc height and allows for bone growth therethrough for the fusion of the adjacent vertebrae. These devices are commonly called “fusion devices.”
Intervertebral body fusion devices typically must carry extremely high loads (on the order of 1-4 kN) for a period of several months, or until fusion occurs. Accordingly, a fusion device or bone graft substitute designed for promoting bony fusion at another location in the body (such as long bone fusion) may not be suitable for use as an intervertebral body fusion device. For example, many bony fusion devices disclose the use of a gel such as a hydrogel as the structural carrier for an osteoinductive or an osteogenic component. However, such gels typically do not posses the stiffness or mechanical strength found to be required for lumbar intervertebral fusion devices.
In general, delivery of conventional intervertebral fusion devices has required significantly invasive implantation procedures. Open surgical implantation of posterior implants requires excision of stabilizing muscles, ligaments, tendons, and bony structures such as the facet joints. The implants must not only overcome the destabilization caused by the surgical procedure, but must add the extra stability needed to promote bony fusion. Open anterior surgery in the lumbar spine is very risky due to the close proximity of sensitive vascular structures, such as the aorta and bifurcation of the aorta. Furthermore, the anterior open procedure can cause significant scar formation on the spine, making anterior revision surgery, if necessary, even more risky.
Minimally invasive procedures have been developed to help mitigate these problems. However, current techniques require appreciable surgical expertise and can significantly increase surgery time. Furthermore, insertion of interbody fusion cages through minimally invasive means often requires high insertion forces.
A number of such prosthetic implants have been described for serving as an intervertebral disc, or nucleus pulpsus, replacement, involving the delivery of prosthetic materials through a small diameter cannula no larger than is needed to perform an adequate discectomy. Therefore, the injectable prosthetic devices are typically delivered in a first fluid form and then harden to a second form once inside the disc space to span the disc space height and preferably fill the disc space following discectomy. However, the requirements for a bone fusion system are very different from those of injectable prosthetic devices.
In summary, there is a need for an intervertebral strut injectable into the disc space that can create or maintain a preferred spatial relationship between adjacent vertebral body endplates (curvature and distraction) and comprises an osteogenic component to promote bony fusion between the two adjacent vertebra.